It seems the game benefits much more from increased cpu performance.

Overclocking your cpu via the fsb, raising ram speed and the northbridge frequency, has the best effect for smoother gameplay/ higher fps.

I would see overclocking the gpu as a supplementary measure, I do it from time to time for this game, but the effect is not as performance enhancing as in other games.

I think my i7920 OC to 3.5 Ghz should be enough for smooth playing the CLOD....I certainly saw performance gain with OC-king my GPU....the trick is you need to OC both and find a "sweet spot" so you wont end up with one OC too high or too low....anyway my gpu is loaded on max and CPU is far away from used 100%....ofcourse this sim need more fine tuning in "optimisation area"....this sim runs fine (on most occasions) on my rig (with latest beta) but thats with slightly reduced landscape and without shadows...